The technology used in modern energy saving lighting devices uses mercury as one of the active components. As mercury harms the environment, extensive research is done to overcome the complicated technical difficulties associated with energy saving, mercury-free lighting.
An approach used for solving this problem is to use field emission light source technology. Field emission is a phenomenon which occurs when a very high electric field is applied to the surface of a conducting material. This field will give electrons enough energy such that the electrons are emitted (into vacuum) from the material.
In prior art devices, a cathode is arranged in an evacuated chamber, typically being a bulb with glass walls, wherein the chamber on its inside is coated with an electrically conductive anode layer. Furthermore, a light emitting layer is deposited on the anode. When a high enough potential difference is applied between the cathode and the anode thereby creating high enough electrical field strength, electrons are emitted from the cathode and accelerated towards the anode. As the electrons strike the light emitting layer, typically comprising a light powder such as a phosphor material, the light powder will emit photons. This process is referred to as cathodoluminescence.
Recent advances in research and development within the area of field emission light sources have made it possible to miniaturize the field emission light source such that it may be manufactured as an in comparison small lighting chip rather than the prior-art bulb shaped field emission light source. An example of a chip based field emission light source is disclosed in WO2016096717, by the same applicant and incorporated in its entirety by reference.
In WO2016096717, the field emission light source is disclosed to be possible to be manufactured in large volumes at low cost using the concept of wafer level manufacturing, i.e. using a similar approach as used by IC's and MEMS. In accordance to WO2016096717, a plurality of field emission light sources each comprises a field emission cathode comprising a plurality of nanostructures formed, a spacer element and a cathodoluminescent anode, all arranged on the same wafer substrate.
Specifically, in accordance to WO2016096717 a large number of field emission light sources are manufactured at the same time on a large glass substrate also referred to herein as a wafer. A plurality of spacer element is subsequently placed so that each spacer element encompasses each field emission cathode with a certain minimum distance between the spacer element wall and the cathode. Lastly a plurality of small glass pieces (usually circular), containing the anodes, are sealed on to the spacers so that for each individual device a cavity is formed. This sealing is done under vacuum. In addition, a getter may be placed inside each cavity in order to maintain the vacuum level for prolonged periods of time. It should be noted that the position of the anode and the cathode in this short description is entirely interchangeable.
In all types of manufacturing it is necessary to perform various tests on each part to verify whether the operation of the part is correct, i.e. meeting a certain specification, or not. Therefore, quality control and test for multiple products is a significant issue, and thus forming a significant part of the cost for manufacturing. When manufacturing components/chips in large volume, it is of course of the highest relevance to optimize also the testing process. The testing process is mainly done in order to identify non-working devices and devices that are not operating to specification at an as early stage as possible to avoid spending further cost and effort on those. It also provides valuable feedback to the manufacturing processes done both prior and after this testing. A typical way of achieving this according to current art is to use a computerized test system which includes the appropriate parts to energize and the parts to be tested and measuring the performance. Typically, probes (needles), connected to a test system, are moved over the surface of the substrate to each device, lowered to make electrical contact with the device and the electrical (and optical if needed) test procedure is performed and the result is typically stored electronically. This method may also be adapted to test several devices in parallel by using multiple probes and may either multiplex the energizing and measuring procedure or use multiple parts in the test system to energize and measure the performance of several devices in parallel. This requires a mechanical movement of the probes in x, y and z directions with relively high speed and high precision. Such equipment is commonly available, but of course at a significant cost in both purchase and maintenance. Further common issues with this method are misalignment of probes, wear of probes, eventually causing contact problems.
Furthermore, in an end user application, there may be a need to use multiple light sources arranged adjacent to each other. Typically, this is simply done by taking individual devices and placing them as required in the application at hand. This will obviously require an electrical connection to each individual device and the individual devices must be separated by some distance for practical reasons, thus occupying further space. Furthermore, in order to simplify the electrical energizing, it is desirable that these multiple devices have quite similar electrical performance, i.e. operate with the same current and voltage. Therefore, devices may be selected for such similar performance.
Accordingly, there would be desirable to identify a process to be used in testing of a field emission light source, specifically manufactured in large volumes using wafer technology.